1. Field of the Invention
This invention relates generally to the field of magnetic resonance imaging systems and more specifically to a quadrature antenna for a magnetic resonance imaging system using elliptical coils.
2. Description of the Related Art
Magnetic resonance imaging ("MRI"), also known as nuclear magnetic resonance ("NMR") imaging, has become a valuable tool as a safe, non-invasive means for obtaining information in the form of images of objects under examination. For example, MRI can be used as a medical diagnostic tool by providing images of the whole or selected portions of the human body without the use of X-ray photography.
MRI systems take advantage of the magnetic properties of spinning nuclei of chemical species found in the observed object in the following manner. Each of the nuclei has an internal spin axis and a magnetic pole aligned with the spin axis. The magnetic pole is a vector quantity representing the magnitude and direction of the magnetic field of the nucleus. Application of an external static magnetic field B.sub.o causes the magnetic poles to align themselves along the external magnetic field lines.
The MRI system disturbs this alignment by transmitting an electromagnetic signal to the object. The magnetic field B.sub.1 of this transmitted electromagnetic signal is circularly polarized and is perpendicular to the static magnetic field B.sub.o. This signal causes the nuclei to precess about the external static magnetic field lines. The frequency of this precession typically is in the radio frequency ("RF") range. More specifically, the precession frequency generally lies within a relatively narrow bandwidth of about 1 to 100 kHz at a center frequency of between 1 and 100 MHz.
As the nuclei precess, they radiate an electromagnetic signal having a circularly polarized rotating magnetic field. The frequency of this rotating magnetic field is gererally equal to the precession frequency of the nuclei. The radiated signal is received by the MRI system to produce an image of the object under examination.
The circularly polarized magnetic fields of the transmitted and received RF signals described above rotate in a plane perpendicular to the static magnetic field B.sub.o. For convenience, a rectilinear coordinate system is used here to describe the orientation of these magnetic fields. The static magnetic field B.sub.o is assumed to be in the direction of the Z axis. Therefore, the rotation of the circularly polarized magnetic fields is in the X-Y plane.
Quadrature coil antennas have been used in MRI systems to transmit and receive the RF signals described above. An example of such a quadrature coil antenna is shown in "Quadrature Detection Coils--A further .sqroot.2 Improvement in Sensitivity," C.N. Chen, D.I. Hoult, and V.J. Sank, J. Maqn. Reson., Vol. 54, 324-327 (1983). This antenna includes a cylindrical antenna structure having four saddle coils arranged into a first coil system and a second coil system. The coils of the first coil system are opposite each other. The coils of the second system are also opposite each other and are oriented 90.degree. relative to the first coil system. Each of the coils is physically 120.degree. wide around the lateral edge of the cylindrical antenna structure and, thus, overlaps each of its adjacent coils by 30.degree.. With this arrangement, the first coil system responds to signals along the X axis while the second coil system responds to signals along the Y axis.
The signals received on the first and second coil systems are coherent but 90.degree. out of phase. A transmit/receive circuit coupled to the antenna adjusts the phase difference between the signals so that they are in phase, and combines these signals to produce a single output signal. Noise in the respective coil systems is assumed to be uncorrelated. Therefore, combination of the signals results in an improved signal-to-noise ratio for the MRI system.
A drawback of this quadrature antenna design is the cross-coupling of the coil pairs. A voltage in one of the coil pairs induces a corresponding voltage in the other coil pair. This problem is attributable to the extensive overlap of each of the respective coils with its adjacent coils. It results in correlation of the noise in the respective coil systems or receiving channels of the MRI system and offsets the gain in sensitivity that is attainable with decoupled channels.
Another antenna design which has been used in MRI systems is an elliptical coil antenna. For example, a crossed ellipse coil for use in an MRI system is described in "A Crossed Ellipse RF Coil for NMR Imaging of the Head and Neck," T. W. Redpath and R. D. Selbie, Phys. Med. Biol., Vol. 29, 739-744 (1984). This antenna coil comprises a single conductor wrapped around a cylindrical former for one nearly complete revolution at an inclination of approximately 45.degree. from the longitudinal axis of the former. Each end of the conductor is then turned 90.degree. and wrapped around the former, also at an inclination of approximately 45.degree. with respect to the longitudinal axis of the former. This antenna has a magnetic field polarization axis directed either along or perpendicular to the longitudinal axis of the former, depending on the specific configuration of the antenna.
The performance of this antenna is limited by its inability to respond to more than one direction, i.e., along both the X and Y axes. It can respond to the X or the Y component of the circularly polarized magnetic fields, but not both simultaneously.
Accordingly, it is an object of the present invention to provide an antenna for an MRI system which has reduced cross-coupling between the respective channels or coil systems of the antenna. It is further an object of the present invention to provide an antenna for an MRI system which provides signal response of both the X and Y components (perpendicular to the direction of the static magnetic field) of the circularly polarized magnetic fields of the transmitted and received RF signals.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and ( combinations particularly pointed out in the appended claims.